The present invention relates to a method of making a hollow waveguide for transmitting electromagnetic radiation, such as ultraviolet (UV), visible (VIS), or infrared (IR) radiation.
Flexible waveguides (WG) are used to transmit electromagnetic radiation (UV, VIS or IR) from a source to a target in straight or curved trajectories. However, the transmission of the electromagnetic energy is reduced by the roughness of the waveguide inner walls. Such roughness increases losses by scattering and by absorption of the radiation. The scattering centers are of two types: (1) Ripples and defects in the plastic tube; and (2) microcrystals, voids, cracks and grains in the deposited metallic and dielectric layers.
In order to achieve minimum losses, high transmitted power, and high flexibility (e.g., bending radii of less than 15 cm) simultaneously, the waveguide may be prepared of three concentric components: (1) a relatively thick insulator tube (e.g., plastic, glass or quartz); (2) a metal layer on the inner surface of the insulator tube; and (3) a dielectric film over the metal layer. Component (1) forms a flexible base, while components (2) and (3) form the energy guiding materials. Our U.S. Pat. No. 4,930,863 discloses a hollow waveguide of this type, and its disclosure is incorporated herein by reference.
This type of waveguide can be made by: (a) depositling a metallic layer and then a dielectric overlayer on the inner wall of a metal, ceramic, plastic, glass (diameter less than 0.8 mm) or quartz tube; or (b) depositing the dielectric layer on the inner surface of a metal tube, and a plastic layer on the outer surface of the metal tube. Method (a) enables one to produce a waveguide having relatively high flexibility but suitable for relatively low power transmission; whereas method (b) enables one to produce a waveguide suitable for higher power transmission, but having lower flexibility.
The plastic, glass or metallic tube, and the metallic/dielectric layers, must all be very smooth. Such smoothness can be achieved either prior to the deposition of each layer, during the deposition, or after the deposition, by smoothing methods as described below. For instance, the base tube, made of plastic, may have ripples or defects in the internal surface, and therefore requires a suitable smoothing treatment to reduce its roughness before metal deposition. The roughness accumulates with every deposited layer since each layer replicates the roughness of its underlying layer(s) and adds its own roughness. Such roughness scatters the radiation and reduces transmission.